1. Field of the Invention
The present invention relates to an optical disk substrate which maintains a high reliability for a long term, and an optical information-storage medium comprising the disk substrate.
2. Description of the Related Art
A polycarbonate resin has been widely used as a substrate for an optical information-storage medium such as an audio-disk, a laser disk, an optical disk memory and a magneto-optical disk, from which information may be read out, written to and/or rewritten, by utilizing laser beams, because the polycarbonate resin has an excellent moldability, strength, light-transmissibility, moisture resistance, and so on. Nevertheless, although the polycarbonate resin exhibits such excellent properties, it has disadvantages that it is easily hydrolyzed under an elevated temperature and a high humidity. This results in lowering of a molecular weight and strength, and also whitening of the product or generation of white points in the product.
The optical disk substrate and the optical information-storage medium prepared therefrom, must be able to maintain a high reliability for a long term (e.g., 10 years or more), but it is difficult to meet this requirement, because of a rapid deterioration of the polycarbonate resin by hydrolysis under an elevated temperature and high humidity.
Various attempts have been proposed to increase a corrosion resistance of an information-storage medium made of the polycarbonate resin. For example, Japanese Unexamined Patent Publication (KOKAI) No. 63-97627 discloses a reduction of a remaining chlorine content to 1 ppm or less. Japanese Unexamined Patent Publication (KOKAI) No. 63-257933 discloses a reduction of a remaining free chlorine content to 1 ppm or less and a phosphorus content to 5-10 ppm.
The techniques disclosed in the above Publications can prevent a corrosion deterioration of the information-recording layer to some extent, but these techniques are not relevant to the deterioration of the polycarbonate resin per se.